Stock Option Plan - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of stock for issuance under the plan	3,553,000
Options exercisable period after, years	6 years
Percentage of options granted	20.00%
Duration of options granted, years	10 years
Stock options outstanding, exercise price range, lower	$ 11.03
Stock options outstanding, exercise price range, upper	$ 30.85
Weighted-average assumptions, risk-free interest rate		1.59%
Expected dividend yield		1.69%
Expected life		5 years 11 months 27 days
Expected volatility		26.91%
Weighted-average grant-date fair value of options granted	$ 0.00	$ 7.14	$ 0.00
Total intrinsic value of options exercised	$ 1,738,000	$ 1,958,000	$ 510,000
Total unrecognized compensation cost related to nonvested share-based compensation arrangements granted under the Plan	2,201,000
Weighted-average period of unrecognized compensation cost recognition, years	2 years 3 months 18 days
Total fair value of shares vested	382,000	565,000	423,000
Aggregate intrinsic value of vested stock options	$ 5,225,438
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options exercisable period after, years	2 years